Python:使类可迭代

Question

我继承了一个项目,其中包含许多大类,只有类对象(整数,字符串等).我希望能够检查属性是否存在而无需手动定义属性列表.

是否可以使用标准语法使python 类本身可迭代？也就是说,我希望能够使用for attr in Foo:(或甚至)迭代所有类的属性,if attr in Foo而无需首先创建类的实例.我想我可以通过定义来做到这一点__iter__,但到目前为止我还没有完全掌握我正在寻找的东西.

我通过添加如下__iter__方法实现了我想要的一些:

class Foo:
    bar = "bar"
    baz = 1
    @staticmethod
    def __iter__():
        return iter([attr for attr in dir(Foo) if attr[:2] != "__"])

但是,这并不能完全满足我的需求:

>>> for x in Foo:
...     print(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'classobj' object is not iterable

即便如此,这仍然有效:

>>> for x in Foo.__iter__():
...     print(x)
bar
baz

Answer 1

添加__iter__到元类而不是类本身(假设Python 2.x):

class Foo(object):
    bar = "bar"
    baz = 1
    class __metaclass__(type):
        def __iter__(self):
            for attr in dir(self):
                if not attr.startswith("__"):
                    yield attr

对于Python 3.x,请使用

class MetaFoo(type):
    def __iter__(self):
        for attr in dir(self):
            if not attr.startswith("__"):
                yield attr

class Foo(metaclass=MetaFoo):
    bar = "bar"
    baz = 1

Answer 2

您可以使用遍历类的未隐藏属性for attr in (elem for elem in dir(Foo) if elem[:2] != '__').

一种不那么可怕的拼写方式是:

def class_iter(Class):
    return (elem for elem in dir(Class) if elem[:2] != '__')

然后

for attr in class_iter(Foo):
    pass

Answer 3

这就是我们如何使类对象可迭代.为类提供iter和next()方法,然后你可以迭代类属性或它们的值.如果你愿意,可以保留next()方法,或者你可以在某些条件下定义next()并引发StopIteration .

例如:

class Book(object):
      def __init__(self,title,author):
          self.title = title
          self.author = author

      def __iter__(self):
          for each in self.__dict__.keys():
              yield self.__getattribute__(each)

>>> book  = Book('The Mill on the Floss','George Eliot')
>>> for each in book: each
...
'George Eliot'
'The Mill on the Floss'

此类迭代类Book的属性值.通过为类对象提供getitem方法,可以使类对象可迭代.例如:

class BenTen(object):
    def __init__(self, bentenlist):
        self.bentenlist = bentenlist

    def __getitem__(self,index):
        if index <5:
            return self.bentenlist[index]
        else:
            raise IndexError('this is high enough')

>>> bt_obj = BenTen([x for x in range(15)])
>>>for each in bt_obj:each
...
0
1
2
3
4

现在当BenTen类的对象用于for-in循环时,getitem被调用具有更高的索引值,直到它引发IndexError.